I. Field of the Invention
The apparatus of this invention is a microwave filter having particular application in transmitters and receivers designed to meet difficult requirements of minimum size, minimum weight and tolerance of extreme environmental conditions. Filters according to the teachings of the present invention are thus suited to use in mobile, airborne, or satellite communication systems in which the requirement exists to sharply define a number of relatively narrow frequency bands or channels within a relatively broader portion of the frequency spectrum. Thus, filters designed according to the present invention are especially useful in bandpass configurations which define the many adjacent channels utilized in satellite communication stations for both military and civilian purposes.
Such satellite communication stations have come to be used for a variety of purposes such as meteorological data gathering, ground surveillance, various kinds of telecommmunication, and the retransmission of commercial television entertainment programs. Since the cost of placing a satellite in orbit is considerable, each satellite must serve as many communication purposes and cover as many frequency channels as possible. Consequently, the ability to realize complex and sophisticated filter functions in compact and lightweight filter units is a significant advance which permits the extension of frequency band coverage without an increase in size or weight. Moreover, these advances are possible without relaxing the stringent requirements which must be met by such communication systems, including the requirement to maintain stable performance over a wide range of temperature.
II. Description of the Prior Art
U.S. Pat. No. 3,205,460 issued Sept. 7, 1965 to E. W. Seeley et al and covers a microwave filter formed of rectangular waveguide dimensioned to be below cutoff at the frequencies for which the filter is designed. However, a rectangular slab of dielectric extends from top to bottom of the waveguide at spaced intervals along the midplane line of the waveguide, such that a series of spaced susceptances is produced. Tuning screws were used to permit fine tuning of the filter. However, this patent contains no information concerning how to realize filter functions more complex than the simple iterative bandpass design which has been illustrated. In particular, there are no teachings as to how to employ dual mode operation, or as to ways to realize cross-couplings for filter designs which require them.
U.S. Pat. No. 3,475,642 issued Oct. 28, 1969 to A. Karp et at, and covers a slow-wave structure in which a series of spaced discs of rutile ceramic extend along a waveguide. The patent contains no teachings of the advantages of using dual mode operation, and employs single mode operation in the TE.sub.01.delta. mode.
U.S. Pat. No. 3,496,498 issued Feb. 17, 1970 to T. Kawahashi et al, and covers a microwave filter in which a series of metal rods, each being dimensioned to be a quarter wavelength long at the frequencies of interest, is spaced along a waveguide structure to form the filter. The rods may be grooved to vary their electrical length without changing their physical length.
U.S. Pat. No. 4,019,161 issued Apr. 19, 1977 to Kimura et at., covering a temperature-compensated dielectric resonator device utilizing single-mode operation in the TE.sub.01.delta. mode.
U.S. Pat. No. 4,027,256 issued May 31, 1977 to Samuel Dixon, and covers a type of wide-band ferrite limiter in which a ferrite rod extends axially along the center of a cylindrical dielectric structure and through the centers of a plurality of dielectric resonator discs which are spaced along the resonant structure. The patent contains little of interest to the worker seeking to realize microwave filter functions in compact high performance filter units.
U.S. Pat. No. 4,028,652 issued June 7, 1977 to Wakino et al., and covers a single-mode filter design in which a variety of differently shaped and dimensioned ceramic resonant elements are disclosed and described. The patent does not, however, suggest the use of dual-mode operation of any of the resonant structures.
U.S. Pat. No. 4,142,164 issued Feb. 27, 1979 to Nishikawa et al., and covers a dielectric resonator utilizing the TE.sub.01.delta. mode. The patent is primarily intended to cover the technique of fine tuning by the application of selected amounts of a synthetic resin which bonds to the ceramic resonator elements to incrementally alter their resonant frequencies. There is no suggestion to use dual-mode operation.
U.S. Pat. No. 4,143,344 issued Mar. 6, 1979 to Nishikawa et al., covering a microwave resonant structure which utilizes two modes in its operation. However, the modes utilized, using the nomenclature of this reference, are the H.sub.01.delta. and E.sub.11.delta., modes which have very dissimilar field distributions. At least partly as a consequence of this fact, the reference contains no teachings as to how to control coupling to each of the modes, and therefore does not show how to realize one pole of a filter function with each of the modes. As a result, there would be no way within the teachings of this patent to realize a complex 6-pole response in a filter having only 3 resonators, as could be done if coupling to each of the modes could be independently controlled.
U.S. Pat. No. 4,184,130 issued Jan. 15, 1980 to Nishikawa et al., and covers a filter design employing a single mode (TE.sub.01.delta.) in a resonator which is coupled to a coaxial line by means of a short section of that line which has been made leaky by cutting apertures in the outer conductor.
U.S. Pat. No. 4,197,514 issued Apr. 8, 1980 to Kasuga et al., covering a microwave delay equalizer. There is no suggestion as to how to make miniature high performance filters which can realize complex filter functions.
In addition to the above prior art which utilizes solid, high dielectric constant resonant elements, there is a considerable body of generally earlier prior art in which unfilled cavity resonators of a variety of configurations were employed, sometimes with dual-mode operation. However, due to the unity dielectric constant of the resonant space, the resultant structures were relatively bulky.
Among this body of prior art relating to unfilled cavity resonators may be mentioned:
U.S. Pat. No. 3,697,898 to Blachier et al. PA1 U.S. Pat. No. 3,969,692 to Williams et al. PA1 U.S. Pat. No. 4,060,779 to Atia et al. PA1 British Pat. No. 1 133 801 to G. Craven.
The Williams et al. patent discusses dual mode filters utilizing the conventional cavity resonators, while the British patent utilizes evanescent modes. However, none of this prior art relating to unfilled cavity resonators contains any suggestion to significantly reduce the volume of the resonant structure by employing resonator element of high dielectric constant as the principal component of the resonator, while enclosing this element within a reduced-dimension cavity which would itself be below cutoff at the frequencies of interest were it not for the included resonator element.